Generally, a vehicle is provided with an obstacle detection device for detecting an obstacle near the vehicle by ultrasonic sonars which are mounted to a bumper of the vehicle. The obstacle detection device will generate an alarm for a driver of the vehicle when the obstacle is detected. In this case, with reference to JP-2003-63335A, the time which elapsed from the sending of an ultrasonic transmitting wave by the ultrasonic sonar to the receiving of a reflection wave from the obstacle as a receiving wave is measured. Moreover, the distance between the vehicle and the obstacle is calculated based on the measured time.
In the case where this distance is smaller than a predetermined value, the alarm is generated. In this case, the ringing manner of the alarm is changed in accordance with the distance between the vehicle and, the obstacle, for example. Thus, it is more necessary for the driver to be called attention to the obstacle with the obstacle being nearer the vehicle.
For example, in the case where there exists an obstacle within a first field (that is, distance between obstacle and bumper is smaller than first predetermined value), the alarm of a continuous sound is generated. In the case where there exists an obstacle within a second field (that is, distance between obstacle and bumper is larger than first predetermined value and smaller than second predetermined value), the alarm of an intermittent sound is generated. The second predetermined value is larger than the first predetermined value. In following, the first field and the second field are respectively called a continuous sound field and an intermittent sound field.
In this case, the multiple ultrasonic sonars are arranged close to each other, in order that the distance of the boundary between the continuous sound field and the intermittent sound field from the vehicle becomes even. As described, different ringing manners are respectively provided at two sides of the boundary. Thus, as shown in FIG. 7, the continuous sound fields of the ultrasonic sonars are continuous with each other, so that the distance of the boundary (that is, border of continuous sound field) between the continuous sound field and the intermittent sound field from the vehicle is substantially equal to the first predetermined value, and the distance of the border of the intermittent sound field from the vehicle is substantially equal to the second predetermined value.
However, in this case, because the multiple ultrasonic sonars are arranged close to each other, the two adjacent ultrasonic sonars are respectively set to have a sending/receiving mode to send the transmitting wave and receiving the receiving wave and a receiving mode to only receive the receiving wave (that is, sending is not performed). Thus, the generation timing of the transmitting wave from the ultrasonic sonars are deviated from each other. The obstacle is detected according to the following procedure. That is, after the detection of the distance between the vehicle and the obstacle based on the transmitting wave sent by the one of the ultrasonic sonars is finished, the another one of the ultrasonic sonars adjacent to the one thereof is subsequently made send the transmitting wave so that the detection of the distance of the obstacle is similarly performed. Thus, it can be discriminated with respect to the transmitting wave of which of the ultrasonic sonars the ultrasonic sonars receive the receiving wave, while the distance of the obstacle can be detected by the multiple ultrasonic sonars.
However, as shown in FIG. 7, at the part (which is surrounded by broken line in FIG. 7) where the continuous sound fields of the adjacent ultrasonic sonars which border each other, the distances between the vehicle and the positions at the border of the continuous field becomes uneven. That is, the obstacle detection distance of the obstacle detection device becomes uneven.